1. Field of the Invention
The present invention relates to semiconductor technology, in particular to a method of forming a silicon nitride layer.
2. Description of the Related Art
Silicon nitride (Si3N4), a popular dielectric material in semiconductor fabricating process, is commonly used in, for example, masking layers, stop layers or passivation layers on integrated devices because of either its ability to protect against the diffusion of impurities and water or a predetermined mechanical strength it performed.
Normally, silicon nitride can be formed by a chemical vapor deposition (CVD) process under low pressure or a plasma enhanced chemical vapor deposition (PECVD) process.
Presently, silicon nitride is formed by reacting dichlorosilane and ammonia at reduced pressure (for example between 0.1 to 10 Torr.) and a temperature ranging from 700 to 800° C. through LPCVD process to achieve better uniformity.
In addition, silicon nitride of PECVD process can be formed by reacting silane, ammonia and nitrogen at lower temperature (normally below 450° C.) and contains hydrogen ranging from 7 to 30% therein. Thus, its stoichimetry is shown as SiNx and differs from that formed by LPCVD.
Thus, in semiconductor fabricating process, thicker silicon nitride layer for masking layer or stop layer of chemical mechanical polishing (CMP) or etching is normally formed by LPCVD. Thicker silicon nitride layer for passivation layer of back end of line (BEOL) process can be formed by PECVD at a lower process temperature.
In addition, another method for forming silicon nitride layer can be thermal nitridation in an ambient of nitrogen atoms at temperature exceeding 1000° C. to nitridize the exposed silicon surface on a substrate with the ambient nitrogen atoms, thus forming a silicon nitride layer. The thermal nitradation, for example, can be a furnace nitridation process or a rapid thermal nitridation (RTN) process. Silicon nitride layer formed by a conventional thermal nitridation process is illustrated by a schematic cross-section shown in FIG. 1.
In FIG. 1, a semiconductor substrate 10, for example a silicon substrate, is provided. Plan surface and an opening OP on and within the substrate 10 expose silicon surfaces thereof. Next, a thermal nitridation process (not shown) is performed. During the thermal nitridation process, the substrate 10 is heated to a predetermined temperature by heaters in the ambient environment (not shown) and then a nitrogen-containing gas G is introduced and decomposed at high ambient temperature to liberate nitrogen atoms therefrom. Thus, the nitrogen atoms can react with the exposed silicon surfaces of the plan surface and the opening OP to form a silicon nitride layer 12 thereon.
Nevertheless, when a thin silicon nitride layer is formed on the substrate 10, because of the dense crystalline structure of the formed silicon nitride layer, the nitrogen atoms in ambient environment become difficult to diffuse and react with the silicon atoms in the deeper portion of the surface of the substrate 10. Thus, the thickness of the silicon nitride layer 12 is restricted by the subsequent mass transfer issue of the thermal nitridation process. Normally, the thickness of the silcon nitride layer 12 formed by thermal nitridation is about 20 Å and cannot be increased by increasing the reaction time or the process temperature of the thermal nitridition process.
In table 1, results of tests forming silicon nitride layer on a plane silicon wafer by thermal nitridation at a fixed flow rate (about 20 SCCM) of a nitrogen-containing gas (N2 here) are shown. The thickness of a silicon nitride layer formed by rapid thermal nitridation (RTN) process and the process conditions thereof are also shown in table 1.
TABLE 1Process temp. (° C.)Process time (sec)thickness (Å)10503417.410509018.911009020.5
As shown in table 1, an unobvious increase (less than 2 Å) of the formed silicon nitride layer thickness is found by the elevated process temperature (about 50° C.) or the process time (about double the original process time) and supports the described theory.